Sorry, don't ask, I solved the "scribble not on full-screen" issue. OMG, the onyx programmers are just plain stupid. There are by default four templates, which can be seen from the firmware update archive in the /notes_template folder. Here comes the clue: the images are resized to fit the M90's screen. That's no big deal, but then you can't simply put 600x800 template images in that directory, but something with an aspect ratio of 768x1200 (or 768x1170, since the bottom tool bar can't be removed, it seems).
So, in order to fix this: download newer firmware, put a blank png file with a resolution of 768x1170px into the /notes_template folder and do the updating procedure. Then scribbling works in full-screen:
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It's those sorts of plain stupid usability issues that the M90 has
